Re: [PATCH v2 1/2] sched/fair: Take thermal pressure into account while estimating energy

[Lukasz Luba <lukasz.luba@arm.com>]

On 6/10/21 10:41 AM, Vincent Guittot wrote:
> On Thu, 10 Jun 2021 at 11:36, Lukasz Luba <lukasz.luba@arm.com> wrote:
>>
>>
>>
>> On 6/10/21 10:11 AM, Vincent Guittot wrote:
>>> On Thu, 10 Jun 2021 at 10:42, Lukasz Luba <lukasz.luba@arm.com> wrote:
>>>>
>>>>
>>>>
>>>> On 6/10/21 8:59 AM, Vincent Guittot wrote:
>>>>> On Fri, 4 Jun 2021 at 10:10, Lukasz Luba <lukasz.luba@arm.com> wrote:
>>>>>>
>>>>>> Energy Aware Scheduling (EAS) needs to be able to predict the frequency
>>>>>> requests made by the SchedUtil governor to properly estimate energy used
>>>>>> in the future. It has to take into account CPUs utilization and forecast
>>>>>> Performance Domain (PD) frequency. There is a corner case when the max
>>>>>> allowed frequency might be reduced due to thermal. SchedUtil is aware of
>>>>>> that reduced frequency, so it should be taken into account also in EAS
>>>>>> estimations.
>>>>>>
>>>>>> SchedUtil, as a CPUFreq governor, knows the maximum allowed frequency of
>>>>>> a CPU, thanks to cpufreq_driver_resolve_freq() and internal clamping
>>>>>> to 'policy::max'. SchedUtil is responsible to respect that upper limit
>>>>>> while setting the frequency through CPUFreq drivers. This effective
>>>>>> frequency is stored internally in 'sugov_policy::next_freq' and EAS has
>>>>>> to predict that value.
>>>>>>
>>>>>> In the existing code the raw value of arch_scale_cpu_capacity() is used
>>>>>> for clamping the returned CPU utilization from effective_cpu_util().
>>>>>> This patch fixes issue with too big single CPU utilization, by introducing
>>>>>> clamping to the allowed CPU capacity. The allowed CPU capacity is a CPU
>>>>>> capacity reduced by thermal pressure signal. We rely on this load avg
>>>>>> geometric series in similar way as other mechanisms in the scheduler.
>>>>>>
>>>>>> Thanks to knowledge about allowed CPU capacity, we don't get too big value
>>>>>> for a single CPU utilization, which is then added to the util sum. The
>>>>>> util sum is used as a source of information for estimating whole PD energy.
>>>>>> To avoid wrong energy estimation in EAS (due to capped frequency), make
>>>>>> sure that the calculation of util sum is aware of allowed CPU capacity.
>>>>>>
>>>>>> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
>>>>>> ---
>>>>>>     kernel/sched/fair.c | 17 ++++++++++++++---
>>>>>>     1 file changed, 14 insertions(+), 3 deletions(-)
>>>>>>
>>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>>> index 161b92aa1c79..1aeddecabc20 100644
>>>>>> --- a/kernel/sched/fair.c
>>>>>> +++ b/kernel/sched/fair.c
>>>>>> @@ -6527,6 +6527,7 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
>>>>>>            struct cpumask *pd_mask = perf_domain_span(pd);
>>>>>>            unsigned long cpu_cap = arch_scale_cpu_capacity(cpumask_first(pd_mask));
>>>>>>            unsigned long max_util = 0, sum_util = 0;
>>>>>> +       unsigned long _cpu_cap = cpu_cap;
>>>>>>            int cpu;
>>>>>>
>>>>>>            /*
>>>>>> @@ -6558,14 +6559,24 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
>>>>>>                                    cpu_util_next(cpu, p, -1) + task_util_est(p);
>>>>>>                    }
>>>>>>
>>>>>> +               /*
>>>>>> +                * Take the thermal pressure from non-idle CPUs. They have
>>>>>> +                * most up-to-date information. For idle CPUs thermal pressure
>>>>>> +                * signal is not updated so often.
>>>>>
>>>>> What do you mean by "not updated so often" ? Do you have a value ?
>>>>>
>>>>> Thermal pressure is updated at the same rate as other PELT values of
>>>>> an idle CPU. Why is it a problem there ?
>>>>>
>>>>
>>>>
>>>> For idle CPU the value is updated 'remotely' by some other CPU
>>>> running nohz_idle_balance(). That goes into
>>>> update_blocked_averages() if the flags and checks are OK inside
>>>> update_nohz_stats(). Sometimes this is not called
>>>> because other_have_blocked() returned false. It can happen for a long
>>>
>>> So i miss that you were in a loop and the below was called for each
>>> cpu and _cpu_cap was overwritten
>>>
>>> +               if (!idle_cpu(cpu))
>>> +                       _cpu_cap = cpu_cap - thermal_load_avg(cpu_rq(cpu));
>>>
>>> But that also means that if the 1st cpus of the pd are idle, they will
>>> use original capacity whereas the other ones will remove the thermal
>>> pressure. Isn't this a problem  ?  You don't use the same capacity for
>>> all cpus in the performance domain regarding the thermal pressure?
>>
>> True, but in the experiments for idle CPUs I haven't
>> observed that they still have some big util (bigger than _cpu_cap).
>> It decayed already, so it's not a problem for idle CPUs.
> 
> But it's a problem because there is a random behavior : some idle cpu
> will use original capacity whereas others will use the capped value
> set by non idle CPUs. You must have consistent behavior across all
> idle cpus.
> 
> Then, if it's not a problem why adding the if (!idle_cpu(cpu))

To capture the signal value from a running CPU, which then I pass
into the em_cpu_energy() in path 2/2. My apologies for confusion,
this can be just local variable for patch 1/2.

I can create the _cpu_cap as local variable inside this loop,
just for this patch. Then in patch 2/2 I will remove it and define
above the loop, to be available for the call to
em_cpu_energy().